1. Field of the Invention
The present invention relates to a pressure measurement apparatus for measuring a fluid pressure. More particularly, the present invention relates to a pressure measurement apparatus having a semiconductor pressure transducer that is reverse mounted to a carrier substrate.
2. Background Art
Medical applications, complex manufacturing processes, automotive operations and a myriad of related areas require various fluid pressures to be monitored in a manner that is reliable, accurate, low in cost and easy to implement. Typically, pressure transducers are utilized to provide this function. These transducers are devices that convert the physical variable--pressure--into a corresponding electrical signal.
A standard pressure transducer is usually comprised of a thin diaphragm which is capable of being deflected by an applied pressure, the magnitude of which is to be measured. A mechanism is then provided for measuring the amount of deflection of the diaphragm, the degree of deflection being representative of the magnitude of the applied pressure. This measuring mechanism is usually made up of suitable electronic circuitry which is configured so as to generate an electrical signal that is reflective of the pressure exerted on the diaphragm. The electrical signal is then supplied to a monitoring device, which then displays, or otherwise responds to, the applied pressure.
Typically, the electronic measuring mechanism used to measure the deflection of the diaphragm is comprised of a resistive strain gauge that is disposed on the diaphragm. The resistive strain gauge is made up of a combination of resistive wires that are usually arranged in a standard Wheatstone Bridge configuration. The wires are then stretched (or compressed, depending on how they are oriented on the diaphragm) when the diaphragm is deflected by the pressure force. In compliance with standard electrical principles, this tensile force causes the electrical resistance of the wires to change. Thus, by applying a constant voltage across the circuit, any change in the wires' resistance will result in a corresponding change in the electrical current through the wires in accordance with Ohm's law. This electrical current is representative of the amount by which the diaphragm has been deflected, and can be used to determine the magnitude of the applied pressure.
Increasingly, pressure transducers of this type have been constructed using standard semiconductor fabrication techniques. The goal has been to produce transducers that are reduced in size, that are more accurate and reliable and that are more easily manufactured in mass quantities, thereby reducing individual transducer costs. Transducers of this type have the required resistive elements formed on the surface of a single, bonded silicone chip, by using conventional ion implanting or diffusion techniques. A portion of the chip is then etched away by using chemicals or other standard etching methods, so as to form a thin diaphragm. When a pressure is applied to this diaphragm, the resistances of the diffused resistors vary by the piezoresistive effect in proportion to the applied pressure. Thus, a single silicon chip can be formed to comprise both the pressure diaphragm and the measuring circuitry of a pressure transducer.
This approach to constructing a pressure transducer from a semiconductor, although advantageous in many respects, also creates additional problems which must be addressed. For example, in such devices, the semiconductor transducer chip must be physically mounted on a chip carrier in a manner such that the diaphragm can be coupled to the fluid pressure that is to be measured. At the same time, the transducer chip must be electrically connected to the chip carrier in a manner such that the necessary voltage may be supplied to the resistive elements and in a manner such that the resultant pressure signal can be accessed by the monitoring equipment.
Also, the accuracy of a semiconductor transducer is temperature-dependent. Thus, the output of the transducer must be temperature compensated via some type of temperature compensation circuitry. Similarly, if the transducer semiconductor (especially if constructed from silicon) is exposed to light, transient variations can be caused in the pressure readings. Therefore, the semiconductor should be somehow shielded from excessive light exposure.
The accuracy of a semiconductor pressure transducer is also affected by any mechanical stresses that are inadvertently applied to the semiconductor chip. Such stresses can cause transient variations in the pressure reading, thereby decreasing the operational reliability of the transducer apparatus. Thus, the semiconductor pressure transducer must be mounted to the chip carrier such that thermal expansion and contraction of the transducer chip, or other external mechanical stresses, are isolated from the transducer diaphragm.
Finally, the diaphragm of the semiconductor transducer is specifically designed to be of a thickness that is responsive and accurate under a predetermined pressure range, which will be dependent upon the environment in which the transducer will be used. For example, under low pressure situations such as blood pressure monitoring, the diaphragm must be thin enough so as to deflect in response to low pressure pulses transmitted through a fluid catheter line. However, the same blood pressure transducer may be inadvertently subjected to a high transient pressure. Such might be the case where the patient is simultaneously undergoing certain medical procedures--such as an angiogram --which cause a high transient pressure to be present at the pressure transducer due to high injection rates and pressure. Under such circumstances, the blood pressure monitor must be isolated or the diaphragm can be ruptured and the transducer rendered inoperative. Thus, the semiconductor transducer that is designed for a specific pressure range is susceptible to being damaged if it is inadvertently subjected to transient pressures that exceed a critical maximum pressure.
In the past, pressure measurement apparatuses utilizing semiconductor pressure transducers have not addressed each of these characteristics without also diminishing the advantages that are originally sought when using a semiconductor transducer--ease of manufacturing, miniaturization, reliability, and low cost. This can best be appreciated by referring to FIG. 1, which shows a standard implementation of a pressure sensing apparatus using a semiconductor pressure transducer. In prior art devices the pressure transducer chip 2 is mounted to a chip carrier 4 in a manner so as to be positioned directly over a pressure port 6 that is formed through the chip carrier 4. Solder pads 8 (or similar bonding techniques, such as a gold ball bonds or aluminum wedge bonds) are provided on the chip carrier 4 and bonding wires 10 are used to electrically interconnect the transducer chip 2 and the solder pads 8.
As will be appreciated, this electrical interconnect scheme--bonding wires and solder pads--results in a variety of disadvantages. First, a bonding wire and solder pad assembly requires additional surface area to implement, which results in a pressure sensing apparatus that is less conducive to the high-density packaging implementations that are so desirable in a number of applications. Further, as is well known, solder runs like water when overheated, and is brittle and unreliable when applied with an incorrect heat. Consequently, solder pads and bonding wires must be applied via a complex and precise manufacturing process, which adds to the overall cost of the pressure sensing apparatus and increases the chances for manufacturing defects. Solder pads are also less reliable, and may fail over time--especially in extreme environments. Similarly, the bonding wires are difficult to assemble in an automated manufacturing process because they must be placed on the chip one at a time. Further, bonding wires are extremely fragile and are thus subject to breakage during the manufacturing process. Also, the bonding wires are susceptible to breakage once the chip is assembled if the chip is dropped or otherwise subjected to similar forces. Consequently, when connected to the top surface of the transducer chip, the bonding wires must be protected, which is typically accomplished by covering the wires and the top surface with a silicone gel, or similar cap assembly. Again, this is an additional manufacturing step which adds overall cost to the pressure sensor. Finally, the bonding wires themselves can transmit mechanical stresses from the chip carrier to the transducer chip diaphragm and thereby further decrease the overall accuracy of the transducer chip.
As discussed above, a semiconductor pressure transducer must be mounted to the chip carrier such that thermal expansion and contraction of the transducer will not subject the pressure transducer to mechanical stresses which might cause incorrect transient pressure readings. Previously, this was accomplished by mounting the transducer chip on a pedestal (shown as item 12 in FIG. 1) constructed from a material having similar thermal characteristics as the transducer chip, such as pyrex. In this manner, the pedestal 12 will contract and expand with variations in temperature in substantially the same manner as the chip and thereby minimize any mechanical stress which may be caused by the thermal expansion and contraction. However, this adds an additional manufacturing step and increases the cost of the pressure sensing device. Further, the pedestal 12 does not completely solve the problem of isolating the transducer chip from external mechanical forces that may occur from a flexing or bending of the chip carrier. Because the chip is not oriented so as to be coplanar with the substrate, the chip is more susceptible to external forces that may cause inaccurate transient pressure readings.
As is further illustrated in FIG. 1, in prior art pressure transducers the fluid pressure is transmitted to the transducer diaphragm through a pressure port 6 formed through the chip carrier 4. This physical orientation results in several disadvantages. First, the pressure forces are applied against the transducer 2 so as to urge it away from the chip carrier 4. Thus, if the pressure sensing apparatus is used in a high pressure environment, the transducer chip 2 can potentially be separated from the chip carrier 4 if the adhesive used to hold the chip 2 to the carrier 4 fails. In addition, if the transducer diaphragm is subjected to a high transient pressure, there is the potential for the thin diaphragm to rupture, or otherwise fail because there is nothing to prevent the diaphragm from being displaced past a critical point. As discussed above, in the medical environment, the transducer in a blood pressure monitor may inadvertently be subjected to such a pressure, thereby causing the thin diaphragm to be ruptured or otherwise damaged. Obviously, this renders the transducer inoperable, and medical personnel are at least temporarily deprived of critical blood pressure information. Because semiconductor transducers are physically oriented in the manner shown in FIG. 1, pressure measurement devices in the prior art are subject to these types of failures and are thus less reliable than is desired for medical, as well as many other, applications.
Finally, because prior art semiconductor transducers are mounted on the top side of the chip carrier in the manner shown in FIG. 1, they require additional structure to shield the semiconductor transducer from exposure to light. Typically, this has been accomplished by enclosing the transducer within an opaque housing (14 in FIG. 1) that is attached to the chip carrier 4. Again, this adds additional complexity to the manufacturing process and also results in a device having a larger overall size.